2-3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline and Acidosis

A brief global ischemic insult to the brain leads to a selective degeneration of the pyramidal neurons in the hippocampal CA1 region while the neurons in the neighbouring CA3 region are spared. The reason for this difference is not known. The selective vulnerability of CA1 neurons to ischemia can be reproduced in vitro in murine organotypic slice cultures, if the ion concentrations in the medium during the anoxic/aglycemic insult are similar to that in the brain extracellular fluid during ischemia in vivo. As acidosis develops during ischemia, we studied the importance of extracellular pH for selective vulnerability. We found that cell death in the CA1 and CA3 regions was equally prevented by removal of calcium from the medium or following blockade of the N-methyl-D-aspartate (NMDA) receptor by D-2 amino-5-phosphonopentanoic-acid (D-APV). On the other hand, damage to the CA3 neurons markedly decreased with decreasing pH following in vitro ischemia, while the degeneration of CA1 neurons was less pH dependent. Patch-clamp recordings from pyramidal neurons in the CA1 and CA3 regions, respectively, revealed a pronounced inhibition of NMDA-receptor mediated excitatory postsynaptic currents (EPSCs) at pH 6.5 that was equally pronounced in the two regions. However, when changing pH from 6.5 to 7.4 the recovery of the EPSCs was significantly slower in the CA3 region. We conclude that acidosis selectively protects CA3 pyramidal neurons during in vitro ischemia, and differentially affects the kinetics of NMDA receptor activation, which may explain the difference in vulnerability between CA1 and CA3 pyramidal neurons to an ischemic insult.

Topics: Acidosis; Animals; Animals, Newborn; Calcium; Cell Death; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Hippocampus; Hydrogen-Ion Concentration; Ischemia; Mice; Neural Inhibition; Neurons; Organ Culture Techniques; Patch-Clamp Techniques; Quinoxalines; Time Factors; Valine

The new non-NMDA (N-methyl-D-aspartate) receptor antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline) has previously been shown to exert a neuroprotective effect in animal models of cerebral ischemia when administered in the post-ischemic phase. In this investigation the effect of NBQX on acidosis and energy recovery in early reperfusion after 10 min of transient forebrain ischemia with the 2-vessel occlusion model in the rat was studied with 31P NMR spectroscopy. In the intervention group the animals received a bolus dose of NBQX 30 mg.kg-1 i.v. at the start of reperfusion. 31P NMR spectroscopy was used to measure intracellular pH, ATP and phosphocreatine continuously in-vivo during, and after, the ischemic event. The recovery of high energy phosphates and pH was followed during 30 min of reperfusion. Pre-ischemic levels of phosphocreatine were reached after approximately 9-10 min in both groups. Although a slight improvement could be seen in the intervention group there was no significant difference in the rate of recovery between the two groups. ATP reached 90% of preischemic levels after about 8 min without significant difference between the two groups. With respect to the recovery of intracellular pH, no difference could be shown. Our results do not contradict previously published results, but suggest that the potential protective effect of NBQX is not mediated through improved recovery of energy metabolism in early reperfusion.

Topics: Acidosis; Adenosine Triphosphate; Animals; Energy Metabolism; Hydrogen-Ion Concentration; Ischemic Attack, Transient; Magnetic Resonance Spectroscopy; Male; Phosphocreatine; Phosphorus; Prosencephalon; Quinoxalines; Rats; Rats, Wistar; Receptors, AMPA; Reperfusion; Time Factors